To get more precision use sats per million virtual bytes.
To make review easier keep most calls in tests using
`FeeRate::from_sats_per_kwu` and just unwrap. These can likely be
cleaned up later on if we want to.
For `serde` just change the module to `_floor` and leave it at that. The
serde stuff likely needs re-visiting before release anyways.
This is not a generic UNIX timestamp, but rather a MTP restricted to
have values between 500 million and u32::MAX. Most importantly, it is
*not* a blocktime, which is what is implied by its name and
constructors.
Add an integration test the verifies we have serde traits implemented
for all the main crate types (excl. errors and helper structs).
This also acts as a regression test.